There is a widespread need to monitor the fill level of liquid containing vessels, and there are a variety of ultrasonic level monitors adapted for this purpose. However, many ultrasonic level monitors are invasive, requiring placement of some hardware in contact with the liquid inside the tank and requiring a connection to that hardware which breeches the tank. Others merely provide crude or limited information about the fill level, for example by simply determining whether a particular location inside the tank is wet or dry. Still others require hardware placed at more than one location, adding to equipment cost. The ultrasonic monitoring techniques currently available thus fail to provide a reliable, accurate, and non-invasive sensor that is easily installed at a single point on the exterior of the storage tank. More clearly, there is a need for a non-invasive, self-correcting, ultrasonic liquid level monitor (ULLM) that is inexpensive and can be easily installed, maintained and operated. In one form, the present invention meets this need. Other forms of the invention provide additional benefits and advantages in a novel and unobvious manner.